The unique properties of nitroxyls, which have one unpaired electron, have found many applications. Nitroxyls have been used:
in the spin labeling of biologically active molecules (O. H. Griffiths, et al, Accounts Chem. Res. 2, 17 (1969)); PA0 as radical-polymerization inhibitors of chloroprene (M. B. Nieman, et al, Vysokomol. Soedin 8, 1237 (1966)=CA, 65, 17173a (1966)); PA0 of styrene (J. C. Bevington, et al, J. Chem. Soc 1956, 3506); and PA0 of 2-vinylpyridine (I. V. Savinova, et al, Dokl. Akad. Nauk. SSSR 181, 1177 (1968)=CA, 70, 4643e (1969)); PA0 as inhibitors of the thermal and light-induced degradation of polyolefins (U.S. Pat. No. 3,431,233); and PA0 of poly(vinyl chloride) (U.S. Pat. No. 3,547,874). PA0 E.sub.2 and E.sub.4 are independently alkyl of 1 to 5 carbon atoms, or PA0 E.sub.1 and E.sub.2 together or E.sub.3 and E.sub.4 together or both E.sub.1 and E.sub.2 together and E.sub.3 and E.sub.4 together are tetramethylene or pentamethylene, and PA0 di-(1-hydroxy-2,2,6,6-tetramethylpiperidine-4-yl)sebacate PA0 N-(1-hydroxy-2,2,6,6-tetramethylpiperidin-4-yl)-.epsilon.-caprolactam
Hydroxylamines have found utility as stabilizers for polyolefins (U.S. Pat. No. 3,644,278), for polyesters (U.S. Pat. No. 3,408,422), for polyurethanes (spandex) (U.S. Pat. No. 3,926,909) and for conjugated diene polymers (U.S. Pat. No. 3,432,578).
A large number of nitroxyls have been prepared by oxidation of secondary amines with ozone (S. D. Razumovskii et al., Dokl. Akad. Nauk. SSSR 183, 1106 (1969)=CA, 70, 95987j (1969)); with dibenzoyl peroxide (A. M. Feldman et al, French Patent No. 1,360,030); with various peracids (G. Chapelet-Letourneux et al, Bull. Soc. Chim. 1965, 3283); or with tert-butylhydroperoxide (O. W. Maender, et al, J. Org. Chem. 34, 4072 (1969)).
Some limitations of the methods cited were mentioned by the last authors; namely that "Oxidizing systems involving hydrogen peroxide, tert-butylhydroperoxide or m-chloroperbenzoic acid were either too slow or ineffective".
The most useful method reported in the literature for the oxidation of secondary amines to nitroxyls requires the use of aqueous hydrogen peroxide in the presence of pertungstate ion (O. L. Lebedev, et al. Doklady Akad. Nauk. SSSR 140, 1327 (1961); and O. L. Lebedev, et al, CA, 56, 15479f (1962)).
Unfortunately, when the secondary amine is not very soluble in water, the reaction becomes so slow that it is not practical. The use of organic solvents miscible with water can alleviate this problem only if the system remains homogeneous, i.e., the amine does not precipitate or form a distinct phase. (E. J. Rauckman, et al, Synthetic Communications 5, 409 (1975)).
It would be desirable to manufacture nitroxyls by direct oxidation of the amine without the disadvantages of the prior art methods.
A liquid phase method of oxidation of cyclic amines was reported to produce lactams. U.S. Pat. No. 3,634,346 describes the oxidation of a cyclic amine having at least one unsubstituted methylene position adjacent to the ring nitrogen with a hydroperoxide in the presence of a metal ion catalyst.